Chinese
scientists have sequenced the genome of a bacterium that is a leading
cause of infant mortality in developing countries. About one million people
die of Shigella infections every year, most of them children. The
bacterium Shigella flexneri causes sudden and severe diarrhea in
humans, known as shigellosis.

New treatments are needed for this highly infectious microbe because
antibiotics are often inadequate and drug-resistant strains are on the
rise. Currently, no vaccines exist and the World Health Organization considers
the development of a vaccine a priority.

The publication of the genome sequence is an important step achieving
this goal. The researchers identified regions of DNA linked to the virulence
of the organism; these are promising targets for vaccines.

The sequenced S. flexneri strain was isolated from a patient with
severe acute shigellosis in Beijing in 1984. The bacterium is commonly
found in water polluted with human feces. It is transmitted in contaminated
food or water and through contact between people. Upon infection, humans
develop severe abdominal cramps, fever, and frequent passage of bloody
stools.

The bacterium has about 4,700 genes. The S. flexneri genome consists
of a chromosome and a smaller DNA structure called a virulence plasmid,
which contains genes important in causing disease. The plasmid includes
regions that are densely populated with genes called pathogenicity islands.

So far, the scientists identified 64 putative pathogenicity islands.
"We are working to explore what percentage of the islands is involved
in virulence," says Qi Jin of the State Key Laboratory for Molecular
Virology and Genetic Engineering in Beijing, China, who led the study.

As part of the study, the researchers compared the sequenced Shigella
pathogen to E. coli, which was sequenced last year. The comparison
revealed that both microbes may be closely related and even constitute
the same species. The findings are reported in Nucleic Acids Research.

Shigellosis is not only a significant cause of infant mortality in developing
nations but maintains endemic levels of infection worldwide. According
to the US Food & Drug Administration, an estimated 300,000 cases of
shigellosis occur annually in the United States. Polluted water and unsanitary
handling of food are the most common causes of disease.

More information on the S. flexneri genome is available from the
Microbial Genome Center of Chinese Ministry of Public Health here.